Free piston engine



Filed Jan. 23,' 195e 4 Sheets-Sheet. 1

July 12, 1960 T. H. FlKsE l 2,944,535-

FREE PIsToN ENGINE Filed Jan. 23, 1958 4 Sheets-Sheet 2 July l2, 1960 Filed Jan. 23, 1958 T. H. FIKSE FREE PISTON ENGINE 4 Sheets-Sheet 3 July 12, 1960 T. H. FlKsE 2,944,535

FREE PIs'roN ENGINE Filed Jan. 23, 1958 4 Sheets-Sheet 4 y Afro/@wry 2,944,535 FREE PrsToN ENGINE v TymanH. Fikse, Seattle, Wash., assignorftoV Stewart- Warner Corporation, Chicago, IIL', a corporation of Virginia i i Filed Jan.f2'3, 1958Ser. No. v710,663

10V claims-(citiamo) y' This invention relates to, a new and improved free pis; tonengin'e.- j

Y' fIn the usual-opposed free'piston -engineof theprior art-,; a'pair of pistons operate Vopposite each' otherin a horizontallydisposed;cylinder; `A-mixture ofgair and fuel is tired Within a vcentral?:combustion chamber located betweenjthe pistons, and the resulting combustion ofi-the'fuel drives ythe pistons apart; Asth'e'pistons move outwardly, the pistons compress air at the ends -of the cylinderf This compressed air bounces the pistons back to the center of the cylinder so that :the operating cycle iscontinued.

.e The ethciency'of this engine is detrimentallylalected by nonsynchronous movement of thepistons. Accordingly, complicated and cumbersome mechanical-means are usually employed to interconnect the pistons/for. proper coordinated movements; The-incorporationof Vme- Another object is to'v synchronize 'the movements ofA oppositely movable pistons of Van Y:engine so 'that they maintain substantially "corresponding:relationships to their respective cylinderswithoutl mechanical interconnectionofanyform. i 'f 'f operational elciency and simplified 2,944,535 Patented July l2, 196i) l CC 2 vpression of air in the associated auxiliary air Vcompression chamber, initiation of the power stroke for the opposite piston is effectedautomatically. Thus, when each of the 'pistons reaches a critical outermost position, it effects initiation of the power stroke for the other piston.

In order that all of the features for attaining the objects of this invention may be readily understood detailed reference is hereinl made to the drawings wherein:

Fig. 1 is a side elevation Vof the free piston engine of the present invention with the cylinder walls broken away so as to show theY internal construction; i Fig'. 2 'is a simplified; longitudinal sectional view-o the engine shown in Fig. land illustrating thepistons'in one possible operating position;

Figs.' 3 and4 are views -related toFig. 2 and showing the pistons inV other possible operating positions; i Fig. 5 is a simplied 'longitudinal' 'sectional view fof the free piston engineof this invention and employing an alternative type of cross-synchronization control;

Fig. 6 is a side elevation Vof-a Pescara/.type free-piston engine 'modiled to incorporate'a cross-timingsynchronizatio'n arrangement Aof the 'generaltype employed :in the 'engineofFig-l; j 15. J I: Fig. 7 shows a modiication tothe cross-timing synchronization arrangements of Figs. 1 and 6; and Fig.;8 is :a graphshowingthe manner infwhich-variations in the tirneof fuel injection vary ythee-cycling fre- 4chanical Vsynchronizing meansrequires that linkages prol quency of individuall pistons in a free piston engine. f

The free piston engineillustrated yin Fig. l1 is of .the tWo-cylinder-opposed type andincludes an engineorfcylinder. block having two identicalcylinder.;j sections" 1 land an intermediate. compressor section .2 bolted together. The bore inthe central.compressorsection,:is larger than the bores in the cylinder sections.` The free pistons are `Another object is-to provideimproved meansin a j free Apiston engine-for sensing-the position of 'one piston and eifecting initiation ofgthepower'stroke lfor .the other piston when the one piston has reachedY a'particular position. Y f f I VThese objects are attained: herein by an engine arrangement-in which two combustion chambers are disposed at kthe. opposite'ends ofja Ycomposite cylinderformed in an engine block and containing two free pistons. A cornmon main air compression chamber is centrally'disposed i between the adjacentends ofthe pistons..V 'TheburningY of -ueli theendk combstionchambers drives-the pistons, inwardly. As the 'pistons move inwardly, thepis;

tons icompress theairrwthinl'the main'chamber.. This compressed air` bounces the pistons back the cylinder to continuethe'cycle; v

YSynchronization =of the pistons is effected by pneumatic ysensing means that requiresv no connection'to the pistons; "-A preferredI `*synchronizing` arrang'ement,` fgorfY example, employsan auxiliary air compression chamber in theengine block for eachpinston. Air isy compressed in -each chaanber A whenl the "associated piston apprachesjits most outward position.v When a piston has reachedia'preto the ends vof Y housed in the engine block. f Th'esepistonsfeach include abody portion 3 disposed in a cylinder bore gfor'nedin 'an engine block `end section and an enlarged h'ead 4 disposed in the bore of ithe'central compressor section 2. The body and head portions of each:piston'may;have suitable sealingrings of conventional characterzassociated therewith. Y .g`

.The Yengine is preferably of the two-stroke cycle type so that each combustion chamber 5 has inlet and exhaust ports controlled by movement'of Ythe associatedr'piston body 3. Compressor section 2 supplies air toairj casing 24 through delivery valves 20. This air is conveyed to combustion chamber 5 through inlet ports 7. Each 'combustion chamber 5 is exhausted intoma'nifold 19 through exhaust ports -6, thereby energizingfturhine' 1-8 sc .that shaft power is delivered at-16. i v If Ythe engine is of the liquid fuely injectiontype as *illustrated, an injection nozzle 8 injects a stream of atomizedfuelv intothe` combustionv chamber 5 when the injector mechanism is actuated. A'conventional water cooling jacket 9 encircles each combustionfchamber 5. The principal air compressionchamber' 10 is located betweenthe heads 4 of the: two pistons. An auxiliary-air compression chamber 11 .islocated betweeneach Yofthe pistons and. the adjacent cylndersection ofthe engine block.' These auxiliary' Vchambers constitute v`pneumatic sensing mechanism for Idetermining thel lengthwisevv position of Veach piston inthe enginejblock:Ventfpassages 121extend through the-wall of the centralsection.- These V passagesvent the chambers Il /behind the piston heads 14 auxiliary air compression chambers'lll. f

'aetemie'i outward pasaran sensed bythe degree qf com- ,1.

The engine compressesair in the aircompressionfchanrber `10lpetweenthepiston heads 4. This-airisA delivered the piston bodies 3 have moved inwardly sufficiently far to uncover inlet ports 7, the pressure of the air compressed in the air casing 24 provides scavenging forrcornbustion chamber 5.

Chamber 16 has a clearance volume determined by the space between the two rows of delivery valves Zi). This enables return energy to be provided for the pistons so that final compression can occur in combustion chambers 5. As the pistons 3, 4 are driven outwardly by the bounce of the air trapped between the piston heads, the momentum of the pistons carries them to substantially the outer ends of combustion chamber. When the pressure in the compression chamber l@ has been reduced below inlet pressure which may be atmospheric pressure, by the outward movement of the piston heads, additional air is supplied to the air compression chamber l through one or more air inlet ports 13. .Several of these ports are disposed in two separate rows. Air passage through each of these ports is controlled by a check valve i4 which prevents air from being forced back out through a port 13 as the pressure of the air in the chamber lt) is increased by approach movement of the piston heads 4. Screen 15 filters the air flowing through check valves d4.

As mentioned previously, the pressure of the air in air compression chamber is relied upon to drive the pistons 3, 4 outwardly after the exhaust ports 6 have been opened and the pistons have completed their power stroke. Itis important that the pressure of the air in the chamber 10, taking into consideration the proportionately larger size of the piston heads 4 than the piston bodies 3, be suficient to force the pistons outwardly' to close the inlet ports 7 and then to compress the air or combustible mixture in the combustion chamber before the pistons, reach their fully outward positions.

When the piston bodies 3 have been driven outwardly sufficiently far by the residual compressed air in' chamber 10, the power strokes are initiated in the cylinders by burning of the combustible mixture therein. Under ideal conditions the two pistons move outwardly in perfect Synchronization and the combustible mixture is burned simultaneously in both combustion chambers S. If move- 'ment of the pistons is slightly out of synchronism, however, and combustion occurs in both cylinders simultaneously, the piston last to arrive at the outer end of its stroke receives a longer and more powerful force becaues ignition starts slightly before the end of the compression stroke and continues until the exhaust ports are uncovered. The effect, therefore, is for the drive on the lagging piston to cause this piston to catch up to the leading piston.

It is proposed herein to use the action of air compressed in the auxiliary compression chambers 11 to effect initiation of the power strokes. The pressure from one of these auxiliary compression chambers is transmitted through a small conduit 22 to a power stroke initiating device 23. In the particular engine illustrated which is of the liquid fuel injection type, such power stroke initiating device may be Ian actuator for the plunger of the injection nozzle 8. Thus compression of air in an auxiliary air chamber 11, effected by outward movement of a piston beyond the apertures 12, senses the approach of the piston to its maximum outward position.

The left auxiliary compression chamber 11 is con nected by one conduit 22 to the right cylinder power stroke initiating device 23, and the right auxiliary compression chamber 11 is connected by a` second conduit 22 to the left cylinder power stroke initiating device 23. Such an interconnection functions eectively to maintain synchronization of the piston movements without a mechanical connection between the pistons.V

To illustrate particular instances, examples of relative piston positions are shown in Figures 2, 3 and 4. Figure 2, for example, the right power pistonis shown atits extreme outermost position at the instant of reversal of movement. Compression of the air in the right auxilinV yclogged and should free itself.

iary compression chamber 111 has caused the left nozzle to inject notwithstanding the fact that the left piston is lagging and has not yet reached its outermost position. The left piston continues to compress the air and ignition does not occur until slightly before the left piston reaches its outermost position as indicated in Figure 3.

The air compressed in the right combustion chamber 5 by outward movement of the right piston initiates movement of the right piston toward the left, but such movement is not as rapid as it otherwise would be because no fuel has been delivered to the right cylinder t0 effect combustion. When the left piston reaches its outermost position, however, the pressure in the left auxiliary compression chamber 11 effects combustion of fuel into the right cylinder.

Under these circumstances the left lagging piston receives a full power impulse, and the right leading piston receives a comparatively weak impulse because the exhaust ports are opened relatively soon after the fuel is injected. This enables the left piston to catch up with the right piston so that thereafter they Will tend to move in synchronism. The less the pistons are out of phase, of course, the more easily they may be brought into coordinated phase movement by this technique. It will be understood that the situation discussed is unusually exaggerated.

A still more exaggerated situation representing possibly the worst conceivable condition of lack of synchronization is illustrated in Figure 4. In this example, the right piston has moved to its outermost position causing fuel to be injected into the leftcylinder while the exhaust ports 6 lin the left cylinder are still open so that no power impulse can occur. Also it is assumed that the left piston is about to be returned to the right by the slightly compressed air in the left combustion chamber and without closing the ports 6. Y

Under these circumstances, the left piston has not moved far enough to the left so that the air in the left auxiliary compression chamber 11 will effect initiation of the power stroke in the right cylinder 5 by injecting liquid fuel into it. Consequently, the right piston is returned to the left only by the compression of air in the right combustion chamber 5. Under these circumstances, the movement of the right piston to the left will drive the left piston to the left as the air is compressed in main compression chamber 10, and the more rapidly moving right pistony also will be bounced back to the right by the compressed air. By this operation the left piston probably will be driven to the left far enough so that ultimately air will be sufficiently compressed in the left auxiliary compression chamber 11 to initiate the injection of fuel into the right cylinder 5, and the right piston will rebound outwardly to the right far enough to effect injection of fuel into the left cylinder 5. Despite the much greater distance travelled by the right piston over that travelled by the left piston, fuel injection will finally occur in both cylinders. Therefore, Vthe pistons will be driven to approach synchronism, and within a few strokes the pistons should be completely synchronized.

In view of the foregoing explanation as to the manner in which the cross-connections between the auxiliary compression chambers and the power stroke initiating devices induce coordinated and synchronized movement of the two pistons, the question-may be raised as to how the movement of the pistons could ever get out of synchronization as in the examples discussed. Such departure from synchronization could occur if one injection nozzle Yshould suddenly become clogged or partially Likewise, the pistons could lose synchronism when the fuel is shut off `and the engine is stopping if the friction on the two pistons were not identical.

To start the engine it is l.desirable-thatthe two pistons be approximately in the positions shown in Figure 1 in If the pistons 3, 4 are not both inthe same positions relative to their cylinders when the initial puff of Yair is-disc'harged intothe principal compression .chamber 10, a non-synchronized operation such as described above might occur for a shortperiod follownginitiation of theengineoperation.. if l 1 ',InFigureS Va modification is shown in which thev enas-tasse gine block and piston structures are the sarne as those described and the same reference numerals have been employed' where, applicable; In this embfdimenny jhow- ,ever,vitis desired to minimize the responsextime lag resulting from building uppressurerin theV auxiliary-:compression chambers 11 andtransmitting this pressure-to the opposite.. end of the engine- The airrducts22'are therefore' made short, and they are'connecteditopressure actuated circuit. making devicesA 26 whichmay `be conventionalpressure switches. f Each ofy these pressure switches is connected byfconductors M1-toa different solenoid 28 incorporated in a solenoid `fuelvvalve or` soleynoidr'controlled injection mechanism.` A suitableipower supplyi 3,0 Iis connectedin the'r :circuits -to ,energize the ,selenoidslZS when the pressure .switches :26fareclosedg As soon as` the pressure in an auxiliarycompression AVchamber 11 is lsufficient toclose a pressure 'switch 26.it :will renergize the associated solenoidjZSjtoQeffect-injec- The cross-timing arrangement herein comprisesa fuel injector 58"ass`ociated with eacli'combstiolfcli'amber 53. A conduit 59 connects each injector 58 with thel op'- posite combustion chamber 53. Accordingly, the left in'- jector 58 is actuated to initiate injection lin the right com'- bustion chamber 53 and right injector 58 is actuated to initiate injection in the left combustion chamber 53. Synchronization occurs inthe manner hereinbefore de'- scribed with reference to the structure lshown in Fig. l. Fig. 7 shows a modification in the cross-timing' synchronization arrangements employed in the free piston engines of Figs. 1 and 6. `For purposesrof illustration, the modified arrangement is shown in conjunction with the free piston engine ofFig. V6. lItA should 'bejunderstood, however,` that the modification `ist-alsok directly applicable to the lengine constniction of Fvigf'l. j

'In Fig. 7, the conduits 65,co1'1ne'ct`edY toinjector mechanisms 58, lead directlyone to the other..4 ',Likewise, conduits 66, connectedto combustion chambers S3 j (or-"to auxiliary` air compressionl chambers 11-in Eig.' 1'),` lead i `directly' one to the other. Intermediateportions of the tion-and construction ofV the engine is otherwise similar .to-that describedfin relation, to; Figs. L th`roughg4.V if,

Figure 6 showsf a .revised'arrangementtof 2a Pescara typefree piston engineutilizing the hereindisclosedcrosstiming. arrangement. This engine structurecomprises-a central' cylinder portion 41"and twofidventical end c'ylinder jportions 42 of. lesserl diameter. Thecentialfand end cylinder portions carry two doubleidiameter pistons 43.

Thegchamben 44;'loc'ated withinv the centrali-cylinder 41A .andY positioned between the piston heads 45 is utiliaed'as .thebouncespacekv In- 'the conventionaLPescara engine 'this space is'divided at the-extreme ends of the engine. The spaces V46 within the centralV cylinder and between ypiston shoulders 47 and end .walls 4&7` are utilized as primary air cornpression'spaces. vCheck Valves 49 provide entry to these spaces during the inwardA strokes of pistons 43. Check valves 50 provide delivery passagesfor air compressed in chambers 46 during the outwardstrokes of pistons 43. The -air delivered through check valves 50 is retained in air chambers 51.

When Pistonsy 43- are at asufficient inward position to maintain ports 52 open between airchambers 51 and lthe cylinder ends of the conventional4 engine and-the cylinder end portions of Fig. 6 are placed end to end to form the central portion of the conventional engine. The rearrangement shown in Fig. `6 is necessary to provide separate combustion chambers 53 kin which injec tion timing can be independently controlled.

connected Yconduits 65 Vand theconnected conduitstre are fjoined one tothe other by a shortconduit 67. Accordingly, both injectors 58 are actuated )inresponse to a pressure that is the -average of .the pressures present ,incombustion chambers 53 (or auxiliarycharnbers in Fig. 1). This average pressure always exceeds the pres,- sure present in'any combustion chamber 5.3 thaty is derfined by a lagging piston 43.. Accordingly, fuel injection is initiated earlier in this latter chamberthan would otherwise be `the case, and therefore synchronism is attained as -hereinbefore described.

' Fig. 8 shows a graphical representation of the piston forces against time in the engines herein described.Y The Ycurves lof this diagram show the-manner in which. early or ,late fuel 4injection 4:affects the. cyclingN frequency VYof individual pistons in axfreev piston engine having noemechanical synchronizing means,

The `force-time curve for perfect synchronizationis represented by the curve Iabcdeyg. Injection occurs-instantaneouslyv at the inner dead point e. ;=The pressure rise resultingffrom the injected fuel is representedby the ef portion ofthe curve; f j

Since the curves shown Iare force-time curves, the tarea u nder these curves lis representative of an impulseV energy quality. With respect to curve abcdefg, certain areas are related inasmuch as points b, d, g are points of maximum piston velocity and points a, ci f are points ofzero velocity. 1

AFor simplicity in analysis, the effects of friction are neglected in the following discussion; Proceeding from a to b, the area under the curve is equal to the kinetic energy of the piston at b, and thiskinetic energy is, in turn equal to the area4 under the curve from b to the piston zero velocity point c. y

v Proceeding from point c, the area under the-curve from c to d equals the kinetic energy of the piston at a', and this areaalso equals the area under the 'curve from b to f, f being a point of zero piston velocity.

For convenience, the period of the curve is defined as,v

s uch as at e", the resulting cycle period would then be -Inasmuch as all systems have fuel and :ignition delays, the pressure that initiates injection at e is sensed` at a time t earlier than e. The portion of the curve from t to e thus represents the ignition delay.

It should be understood that the above described arrangements are illustrative of the principles of this invention and that numerous other arrangements can be devised without departing from the scope of the invention.

' What is claimed is:

1. A free piston engine comprising an engine block defining a cylinder therein, a pair of opposed free pistons disposed within said cylinder to establish a bounce chamber therebetween and a pair of combustion chambers at opposite ends of said cylinder and means supplying a combustible fuel mixture to said combustion chambers.

2. A free piston engine comprising an engine block defining a cylinder therein, a pair of axially aligned opposed free pistons disposed within said cylinder to establish a compression chamber therebetween and a pair of combustionchanrbers at opposite ends of said cylinder, means supplying fuel to said combustion chambers, and means supplying air to said combustion chambers.

3. A free piston engine comprising an engine block defining a cylinder therein, a pair of axially-aligned opposed free pistons disposed within said cylinder to establish a compression chamber therebetween and a pair of combustion chambers at opposite ends of said cylinder, means supplying fuel to said combustion chambers, means supplying air to said combustion chambers, means exhausting said combustion chambers, and means synchronizing said pistons for motion in lreverse directions relative one another.

4. A free piston engine comprising an engine block defining a cylinder therein, a pair of opposed free pistons disposed within said cylinder to establish a compression chamber therebetween and a pair of combustion charnbers at opposite ends of said cylinder, two fuel injection devices each communicating with a different one of said combustion chambers, and means independently actuating each of said fuel injection devices in response to the predetermined positioning relative to the engine block of the piston communicating with the other combustion chamber.

5. A free piston engine comprising an engine block defining a cylinder therein, a pair of opposed free pistons disposed within said cylinder to establish a compression chamber therebetween and a pair of combustion chambers at opposite ends of said cylinder, and means independently initiating combustion in each of said combustion chambers in response to the predetermined positioning relative to the engine block of the piston communicating with the other combustion chamber.

6. A free piston engine comprising an engine block deiining a cylinder therein, a pair of opposed free pistons disposed within said cylinder to establish a compression chamber therebetween and a pair of combustion ch-ambers at opposite ends of said cylinder, two pressure-actuated fuel injection devices each communicating with a different one of said combustion chambers, means associated with each of the pistons and developing a pressure responsive to the position ofthe associated piston relative to the engine block, and means independently connecting each of the fuel injection devices with the pressure means associated with the piston of the opposite combustion chamber.

l7. A` free piston engine comprising an engine block defining a cylinder therein, a pair of opposed free pistons disposed within said cylinder to establish a compression chamber therebetween and a pair of combustion chambers fat opposite -ends of said cylinder, two pressure-actuated fuel injection ydevices each communicating with a diiferent one of said combustion chambers, means associated with each of the pistons and developing a pressure responsive to the position of the associated piston relative to the engine block, means connecting both of said pressure developing means o-ne to the other to develop an Yintermediate pressure, and means actuating'both of said fuel injection devices in response to a predetermined intermediate pressure.V

8. A free piston engine comprising an engine block defining a composite cylinder therein, a pair of axiallyaligned opposed free pistons disposed within said composite cylinder to establish a compression chamber therebetween and a pair of combustion `chambers at opposite ends of said cylinder, two fuel injection devices each communicating with a different Yone of said combustion chambers, and means independently actuating each of said fuel Vinjection devices in response to the attainment of a pre-determined pressure in the combustion chamber of the opposite piston.

9. A free piston engine comprising an engine block `defining a composite cylinder therein, a pair of axiallyaligned opposed free pistons disposed within said composite cylinder to establisha compression chamber therebetween and -a pair of combustion chambers at opposite ends of said cylinder, two pressure-actuated fuel injection devices each communicating with a different one of said combustion chambers, means independently associated with each of the pistons and defining an auxiliary chamber developinga pressuretherein responsive to the position of the -associated piston relative theV engine block and means independently actuating each of said fuel injection ldevices in response to the attainment of a predetermined pressure in the lauxiliary chamber associated with the opposite piston.

10. A free piston engine comprising an engine block defining a cylinder therein, a pair of axially-aligned opposed free pistons disposed within said cylinder to establish a `compression chamber therebetween and a pair of combustion chambers at opposite ends of said cylinder, means supplying fuel to said combustion chambers, means exhausting said combustion chambers, and means timing the injection of fuel into each combustion chamber in response to the attainment of a pre-determined condition in the opposite combustion chamber.

References Cited in therlile of this patent UNITED STATES PATENTS 

